Scratch Interface (!)
This may just be the coolest interface ever. I thought it was a joke when I first read about it: interact with computers through scratching your fingernail on surfaces. Simply amazing.
From the prolific interface genius that is Chris Harrison. Jump to 3:14 for the best concrete example of the technology in use: controlling your phone with gestures on a normal table with nothing but a stethoscope on it.
Computation at its root is distilled physics, interacting with our everyday physics it can produce pure magic. Think of accelerometers as well, or the now commonplace touch displays.